Stored compressed air management for improved engine performance

ABSTRACT

A method for providing air to a combustion chamber of an engine, the engine including a compressor and a boost tank selectably coupled to an intake manifold. The method includes varying a relative amount of engine exhaust in air pressurized in the boost tank based on engine operating conditions, and discharging the air pressurized in the boost tank to the intake manifold.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/761,051 filed Apr. 15, 2010, the entire contents of whichare incorporated herein by reference for all purposes.

TECHNICAL FIELD

This application relates to the field of motor-vehicle engineering, andmore particularly, to air induction in motor vehicle engine systems.

BACKGROUND AND SUMMARY

A boosted engine may offer greater fuel efficiency and lower emissionsthan a naturally aspirated engine of similar power. During transientconditions, however, the power, fuel efficiency, and emissions-controlperformance of a boosted engine may suffer. Such transient conditionsmay include rapidly increasing or decreasing engine load, engine speed,or mass air flow. For example, when the engine load increases rapidly, aturbocharger compressor may require increased torque to deliver anincreased air flow. Such torque may not be available, however, if theturbine that drives the compressor is not fully spun up. As a result, anundesirable power lag may occur before the intake air flow builds to therequired level.

It has been recognized previously that a turbocharged engine system maybe adapted to store compressed air and to use the stored, compressed airto supplement the air charge from the turbocharger compressor.Accordingly, U.S. Pat. No. 5,064,423 describes a system in whichcompressed air is stored in a boost tank and is dispensed wheninsufficient compressed air is available from the turbochargercompressor.

However, the inventors herein have recognized that other transientcontrol issues may occur during decreasing engine load. For example,when a throttle valve in a boosted engine system closes, the compressedair charge upstream of the throttle valve is released to the atmosphereto avoid compressor surge. This may be done by opening a compressorby-pass valve, for example. Such actions erode fuel efficiency, however,as the mechanical energy used to compress the air charge is wasted whenthe air is released to the atmosphere. Moreover, in engine systemsequipped with low-pressure (LP) exhaust-gas recirculation (EGR), merelyopening the by-pass valve may not adequately prepare the engine forlow-load operation. This is because the intake air charge will bediluted with exhaust gas during mid- to high-load operation. When thethrottle valve closes, this exhaust gas remains trapped behind thethrottle valve. During closed-throttle conditions, however, non-diluted,fresh air may be required for reliable combustion. Thus, even applyingthe applying the approach of U.S. Pat. No. 5,064,423, combustioninstability during closed-throttle conditions may still occur.

The inventors herein have further recognized that a properly configuredcompressed-air management system can be used to address both of thetransient-control issues identified above. Therefore, one embodimentprovides a method for providing air to a combustion chamber of anengine, the engine including a compressor and a boost tank selectablycoupled to an intake manifold. The method comprises varying a relativeamount of engine exhaust in air pressurized in the boost tank based onengine operating conditions, and discharging the air pressurized in theboost tank to the intake manifold. In this manner, air containing anappropriate relative amount of exhaust gas may be stored and latersupplied to the engine intake during closed- and open-throttleconditions. For example, by varying the relative amount of exhaust gasin the stored air, it is possible to provide a desired mixture of storedgas, which may be different depending on whether the gas is to be usedduring open- or closed-throttle conditions.

It will be understood that the summary above is provided to introduce insimplified form a selection of concepts that are further described inthe detailed description, which follows. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined by the claims that follow the detailed description. Further,the claimed subject matter is not limited to implementations that solveany disadvantages noted herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows aspects of an example engine system inaccordance with an embodiment of this disclosure.

FIG. 2 is a graph of compressor boost pressure versus flow rate inaccordance with an embodiment of this disclosure.

FIGS. 3 and 4 schematically show aspects of other example engine systemsin accordance with embodiments of this disclosure.

FIGS. 5-7 schematically show aspects of an example boost tank withprovision for draining condensate in accordance with differentembodiments of this disclosure.

FIG. 8 illustrates an example method for selecting a source for fillinga compressed air boost tank in an engine system in accordance with anembodiment of this disclosure.

FIG. 9 illustrates an example method for operating a pressure pump in anengine system based on driver demand, cold-start emissions reduction(CSER), and deceleration fuel shut off (DFSO), in accordance with anembodiment of this disclosure.

FIG. 10 illustrates an example method for controlling a configurablevacuum/pressure pump in an engine system based on vacuum availabilityand compressed air demand in accordance with an embodiment of thisdisclosure.

FIG. 11 illustrates an example method for supplying compressed air froma boost tank in an engine system in accordance with an embodiment ofthis disclosure.

FIG. 12 illustrates an example method for responding to throttle valveclosure in an engine system in accordance with an embodiment of thisdisclosure.

FIG. 13 illustrates an example method for regulating a supply of air toan intake manifold coupled to main and auxiliary throttle valves inaccordance with an embodiment of this disclosure.

FIG. 14 illustrates a method for providing unidirectional air flowthrough a throttle valve in an engine system in accordance with anembodiment of this disclosure.

FIG. 15 illustrates an example method for draining a compressed airboost tank in an engine system of vehicle in a manner in which anoccupant of the vehicle is unlikely to notice, in accordance with anembodiment of this disclosure.

DETAILED DESCRIPTION

The subject matter of this disclosure is now described by way of exampleand with reference to certain illustrated embodiments. Components,process steps, and other elements that may be substantially the same inone or more embodiments are identified coordinately and are describedwith minimal repetition. It will be noted, however, that elementsidentified coordinately may also differ to some degree. It will befurther noted that the drawing figures included in this disclosure areschematic and generally not drawn to scale. Rather, the various drawingscales, aspect ratios, and numbers of components shown in the figuresmay be purposely distorted to make certain features or relationshipseasier to see.

FIG. 1 schematically shows aspects of an example engine system 10 in oneembodiment. In engine system 10, fresh air is introduced via air cleaner12 and flows to compressor 14. The compressor may be any suitableintake-air compressor—a motor-driven or driveshaft driven superchargercompressor, for example. In engine system 10, however, the compressor isa turbocharger compressor mechanically coupled to turbine 16, theturbine driven by expanding engine exhaust. In one embodiment, thecompressor and turbine may be coupled within a twin scroll turbocharger.In another embodiment, the turbocharger may be a variable geometryturbocharger (VGT), where turbine geometry is actively varied as afunction of engine speed. As shown in FIG. 1, compressor 14 is coupled,through charge-air cooler 18, to throttle valve 20; the throttle valveis coupled to intake manifold 22. From the compressor, the compressedair charge flows through the charge-air cooler and the throttle valve tothe intake manifold. The charge-air cooler may be an air-to-air orair-to-water heat exchanger, for example. In the embodiment shown inFIG. 1, the pressure of the air charge within the intake manifold issensed by manifold air pressure (MAP) sensor 24.

In engine system 10, compressor by-pass valve 26 and fixed flowrestrictor 28 are coupled in series between the inlet and the outlet ofcompressor 14. The compressor by-pass valve may be a normally closedvalve configured to open under selected operating conditions to relieveexcess boost pressure. For example, the compressor by-pass valve may beopened during conditions of decreasing engine speed to avert compressorsurge.

In one embodiment, compressor by-pass valve 26 may be a two-state valvehaving a fully open state and a fully closed state. Therefore, as shownin FIG. 1, fixed flow restrictor 28 is coupled in series with thecompressor by-pass valve. In one embodiment, the fixed flow restrictormay be an orifice-type flow restrictor; in another embodiment, it may bea laminar-flow type flow restrictor comprising one or more laminar-flowelements. In either case, the fixed flow restrictor may be configured sothat when the compressor by-pass valve is opened, sufficient air flow isdischarged from the outlet to the inlet to prevent surge, while stillallowing some boost pressure accumulate at the outlet. Accordingly, thedimensions of the fixed flow restrictor may be chosen to keep as muchpressure downstream of compressor 14 as possible—for rapidre-pressurization—but to keep the compressor out of surge conditions.This scenario is further considered below, with reference to FIG. 2.

FIG. 2 shows an example graph of boost pressure versus volumetric flowrate for an example turbocharger compressor. The graph shows sevencurved segments of constant compressor speed ranging from 60,000revolutions per minute to 200,000 revolutions per minute. Intersectingthe seven curved segments are lines labeled SURGE LINE, ORIFICE, andLAMINAR FLOW ELEMENT.

The line labeled ORIFICE shows the result of by-passing the compressorwith an orifice-type fixed flow restrictor of a given size. The graphdemonstrates that such a flow restrictor coupled in the by-pass flowenables the boost pressure to accumulate while staying a safe distancefrom the surge line of the compressor. With an orifice-type flowrestrictor, the flow rate varies as the square root of the boostpressure. The surge line, in contrast, may exhibit an almost linearrelationship between boost pressure and flow rate. As a result, it isadvisable to use an orifice-type flow restrictor sized to stay well awayfrom the surge line at moderate compressor speed if surge is to beavoided at high compressor speed. This situation manages boost and flowrate to avoid surge but maintains some boost when the by-pass valve isopen.

By contrast, the laminar flow element provides an improvedcharacteristic, as shown in FIG on the line labeled LAMINAR. 2. With thelaminar flow element, the flow rate varies linearly with boost pressure.Therefore, a fixed laminar-flow type flow restrictor can be chosen thatmore closely tracks the surge line over most of the operating speedrange of the compressor. Increasing the air compression rate underby-passed conditions may be advantageous in embodiments where compressedair is stored for later use, as further described hereinafter.

Returning now to FIG. 1, intake manifold 22 is coupled to a series ofcombustion chambers 30 through a series of intake valves 32. Thecombustion chambers are further coupled to one or more exhaust manifoldsections via a series of exhaust valves 34. In the embodimentillustrated in FIG. 1, exhaust manifold sections 36A and 36B are shown.Other embodiments may include more or fewer exhaust manifold sections.Configurations having more than one exhaust manifold section enableeffluent from different combustion chambers to be directed to differentlocations in the engine system, as further described hereinafter.

In one embodiment, each of the exhaust and intake valves may beelectronically actuated or controlled. In another embodiment, each ofthe exhaust and intake valves may be cam actuated or controlled. Whetherelectronically actuated or cam actuated, the timing of exhaust andintake valve opening and closure may be adjusted as needed for desiredcombustion and emissions-control performance. In particular, the valvetiming may be adjusted so that combustion is initiated when a controlledamount of exhaust from a previous combustion is present in one or morecombustion chambers. Such adjusted valve timing may enable an ‘internalEGR’ mode useful for reducing peak combustion temperatures underselected operating conditions. In some embodiments, adjusted valvetiming may be used in addition to the ‘external EGR’ modes describedhereinafter.

FIG. 1 shows electronic control system 38, which may be any electroniccontrol system of the vehicle in which engine system 10 is installed. Inembodiments where at least one intake or exhaust valve is configured toopen and close according to an adjustable timing, the adjustable timingmay be controlled via the electronic control system to regulate anamount of exhaust present in a combustion chamber during ignition. Theelectronic control system may also be configured to command the opening,closure and/or adjustment of various other electronically actuatedvalves in the engine system—throttle valves, compressor by-pass valves,waste gates, EGR valves and shut-off valves, for example—as needed toenact any of the control functions described herein. Further, to assessoperating conditions in connection with the control functions of theengine system, the electronic control system may be operatively coupledto a plurality of sensors arranged throughout the engine system—flowsensors, temperature sensors, pedal-position sensors, pressure sensors,etc.

Continuing in FIG. 1, combustion chambers 30 may be supplied one or morefuels: gasoline, alcohols, diesel, biodiesel, compressed natural gas,etc. Fuel may be supplied to the combustion chambers via directinjection, port injection, throttle valve-body injection, or anycombination thereof. In the combustion chambers, combustion may beinitiated via spark ignition and/or compression ignition in any variant.

In embodiments where fuel is supplied by direct injection, differentcombustion chambers 30 may be charged with unequal amounts of fuelduring selected operating conditions. For instance, engine system 10 maybe configured for a DFSO mode, where some of the combustion chambersreceive no fuel and merely pump the air admitted through theirrespective intake valves. Under such conditions, the engine system maybe configured to store the air pumped and thereby compressed by theunfueled combustion chambers. Accordingly, FIG. 1 shows two-way valve 40coupled to exhaust manifold section 36B. When the combustion chamberscoupled to exhaust manifold section 36B are unfueled due to DFSOoperation, the two-way valve may be positioned to direct the effluent ofthe combustion chambers—i.e., the pumped, compressed air—to a locus ofthe engine system where the air can be stored (vide infra). In thismanner, one or more unfueled combustion chambers of the engine may beused as an air pump—a functional equivalent of other air pumps describedhereinafter. Under other conditions, when the combustion chamberscoupled to exhaust manifold section 36B are fueled, the two-way valvemay be positioned to direct the effluent of the combustion chambers toturbine 16.

As shown in FIG. 1, exhaust from the one or more exhaust manifoldsections is directed to turbine 16 to drive the turbine. When reducedturbine torque is desired, some exhaust may be directed instead throughwaste gate 42, by-passing the turbine. The combined flow from theturbine and the waste gate then flows through exhaust-aftertreatmentstage 44. The nature, number, and arrangement of exhaust-aftertreatmentstages may differ in the different embodiments of this disclosure. Ingeneral, one or more exhaust-aftertreatment stages may include one ormore exhaust-aftertreatment catalysts configured to catalytically treatthe exhaust flow, and thereby reduce an amount of one or more substancesin the exhaust flow. For example, one exhaust-aftertreatment catalystmay be configured to trap NO_(x) from the exhaust flow when the exhaustflow is lean, and to reduce the trapped NO_(x) when the exhaust flow isrich. In other examples, an exhaust-aftertreatment catalyst may beconfigured to disproportionate NO_(x) or to selectively reduce NO_(x)with the aid of a reducing agent. In still other examples, anexhaust-aftertreatment catalyst may be configured to oxidize residualhydrocarbons and/or carbon monoxide in the exhaust flow. Differentexhaust-aftertreatment catalysts having any such functionality may bearranged in wash coats or elsewhere in the exhaust-aftertreatmentstages, either separately or together. In some embodiments, theexhaust-aftertreatment stages may include a regenerable soot filterconfigured to trap and oxidize soot particles in the exhaust flow.

Continuing in FIG. 1, all or part of the treated exhaust from exhaustaftertreatment stage 44 may be released into the ambient via exhaustconduit 46, in which silencer 48 is also coupled. Depending on operatingconditions, however, some treated exhaust may be diverted insteadthrough EGR cooler 50 and EGR valve 52, to the inlet of compressor 14.In this manner, the compressor is configured to admit exhaust tappedfrom downstream of turbine 16. The EGR valve may be opened to admit acontrolled amount of cooled exhaust gas to the compressor inlet fordesirable combustion and emissions-control performance. Thus, enginesystem 10 is adapted to provide external, low-pressure (LP) EGR inaddition to the internal EGR described above. The rotation of thecompressor, in addition to the relatively long LP EGR flow path inengine system 10, provides excellent homogenization of the exhaust gasinto the intake air charge. Further, the disposition of EGR take-off andmixing points provides very effective cooling of the exhaust gas forincreased available EGR mass and improved performance; as shown in FIG.1, the recirculated exhaust traverses exhaust-aftertreatment device 44,EGR cooler 50, as well as charge-air cooler 18.

In engine system 10, compressor 14 is the primary source of compressedintake air, but under some conditions, the amount of intake airavailable from the compressor may be inadequate. Such conditions includeperiods of rapidly increasing engine load, such as immediately afterstart-up, on tip-in, or upon exiting DFSO. In addition, the intake aircharge supplied by the compressor may not always be well-suited forchanging engine-load conditions. The intake air charge may be highlydiluted with EGR, for example, under conditions where fresh air isneeded. Such conditions include abrupt throttle valve closure and/ortip-out, for example.

In view of the issues noted above, engine system 10 includes boost tank54. The boost tank may be any reservoir of suitable size configured tostore compressed air for later discharge. In one embodiment, the boosttank may be configured to store air at the maximum pressure generated bycompressor 14. Various inlets, outlets, and sensors may be coupled tothe boost tank. In the embodiment shown in FIG. 1, pressure sensor 56 iscoupled to the boost tank and configured to respond to the air pressuretherewithin.

In engine system 10, boost tank 54 is selectably coupled to intakemanifold 22. More specifically, the boost tank is configured todischarge compressed air to the intake manifold via boost tank dischargevalve 60. The boost tank discharge valve may be a normally closed valvecommanded to open when a flow of air from the boost tank to the intakemanifold is desired. In the embodiment shown in FIG. 1, pressurerecovery cone 58 is coupled fluidically between the boost tank and theintake manifold. Accordingly, compressed air is conducted through thepressure recovery cone on discharge from the boost tank. The pressurerecovery cone may be any section of conduit having a graduallyincreasing cross-sectional area normal to the direction of flow. Thepressure recovery cone may be installed anywhere between the boost tankand the intake manifold and may be bent into a curved (e.g., nautilus)shape if needed for efficient packing. Compared to the same length ofconduit having a constant cross-sectional area, the pressure recoverycone converts flow energy back to pressure energy during flow conditionsby suppressing flow detachment from the conduit walls. In oneembodiment, pressure recovery cone 58 may have a 15 degree cone angleand may reduce the flow rate of the compressed air from 200 meters persecond to 50 meters per second. Based on known principles of fluiddynamics, this reduction in flow rate may conserve 47 kilopascals ofpressure for air initially pressurized to 200 kilopascals.

In engine system 10, compressed air from boost tank 54 is delivereddownstream of throttle valve 20. In some scenarios, the compressed airmay be delivered when the throttle valve is at least partially open.Therefore, check valve 60 may be coupled upstream of the throttle valveand oriented to prevent the release of compressed air from the boosttank backwards through the throttle valve. In other embodiments, thecheck valve may be omitted and other measures taken to prevent backwardsflow through the throttle valve (vide infra).

As noted hereinabove, the pumping of air by unfueled combustion chambersof the engine during DFSO provides one way to charge boost tank 54 withcompressed air. In the embodiment shown in FIG. 1, two-way valve 40 maybe oriented such that effluent from one or more unfueled cylinders flowsthrough check valve 62 and into the boost tank. The check valve allowscompressed air from exhaust manifold section 36B to be stored in theboost tank but prevents stored compressed air from flowing back to theexhaust manifold section.

A turbocharged engine system may include still other structure to enablea boost tank to be filled under selected operating conditions. In enginesystem 10, for example, boost tank 54 is coupled to compressor 14 viacheck valve 64. The check valve allows compressed air from thecompressor to flow into the boost tank under conditions of highthrottle-inlet pressure (TIP) and to be stored therein, but it preventsstored compressed air from flowing back to the compressor underconditions of low TIP.

Boost tank 54 is further coupled to air pump 66 via check valve 68. Thischeck valve allows compressed air from the air pump to flow into and bestored in the boost tank when the outlet pressure of the air pump ishigh, but it prevents stored compressed air from flowing back to the airpump when the outlet pressure is low. Air pump 66 may be virtually anyair pump of the vehicle in which engine system 10 is installed. In oneembodiment, the air pump may be driven by an electric motor. In anotherembodiment, the air pump may be driven by a crankshaft or other rotatingor reciprocating shaft of the engine system. In another embodiment, theair pump may be driven by a wheel of the vehicle in which the enginesystem is installed. In yet another embodiment, the air pump may be anexhaust-driven or compressor-driven pressure amplifier—i.e., a gas-flowdriven air compressor.

In the particular embodiment illustrated in FIG. 1, air pump 66 isfurther coupled to vacuum manifold 70 of engine system 10 via checkvalve 72. Accordingly, air pump 66 may be configured to operate as avacuum pump under certain operating conditions and as a pressure pumpunder other operating conditions. During both conditions, the air pumpis operated so as to impel air from the side coupled to check valve 72to the side coupled to check valve 68. For vacuum operation, shut-offvalve 74 is opened, and shut-off valve 76 is closed. The vacuum manifoldis thereby evacuated, providing vacuum for braking and other vehicleoperations. In this configuration, the air pump presents minimalmechanical or electrical load on the engine system. For pressureoperation, shut off valve 76 is opened, and shut-off valve 74 is closed.Boost tank 54 is thereby pressurized with air drawn from air cleaner 12.

FIG. 3 schematically shows aspects of another engine system 78 in oneembodiment. Engine system 78 differs from engine system 10 in that boosttank 54 is coupled upstream, not downstream, of throttle valve 20. As aresult, the flow of air into the intake manifold is controlled in thesame manner regardless of how the TIP is established (stored, compressedair from the boost tank or compressed air directly from compressor 14under high-TIP conditions).

FIG. 4 schematically shows aspects of another engine system 80 in oneembodiment. Engine system 80 differs from engine system 10 in that boosttank 54 is coupled to pressure recovery cone 58 via air ejector 82. Theair ejector has a primary inlet coupled to the boost tank via boost tankdischarge valve 60, an outlet coupled to intake manifold 22, and asecondary inlet arranged off of the main flow. When air flows from theprimary inlet through to the outlet, a partial vacuum develops at thesecondary inlet, causing additional air to be drawn in and dischargedfrom the outlet. Although the air charge provided to the intake manifoldwill be lower in pressure than the air discharged from the boost tank,the overall air mass delivered may be significantly greater. Using theair ejector in this manner offers at least two advantages in enginesystems having a boost tank. First, the volume of the boost tank may bedecreased to save space, while maintaining the overall amount ofavailable boost. Second, the time required to fill the boost tank may bereduced in view of its lower volume.

The secondary inlet of air ejector 82 may be coupled to air cleaner 12or to virtually any other air source. In the embodiment shown in FIG. 4,however, the secondary inlet of the air ejector is coupled to the outletof compressor 14 via auxiliary throttle valve 84 in series with checkvalve 86 and charge-air cooler 88. The charge-air cooler may be anysuitable air-to-air or air-to-water heat exchanger, for example. In thismanner, engine system 80 is adapted to take advantage of whatever boostlevel may have accumulated at the outlet of the compressor at the timewhen compressed air is drawn from the boost tank. In other embodiments,the auxiliary throttle valve may be coupled between the outlet of theair ejector and the intake manifold, rather than upstream of thesecondary inlet.

The example above underscores the importance of maintaining a high airpressure in the boost tank for flexibility in responding to transientconditions. Another way to increase the air pressure in the boost tankis to introduce a small amount of volatile liquid fuel in the boosttank, along with the compressed air. Evaporation of the volatile fuelwill increase the total gas pressure within the tank, furthercompressing the stored air.

In the various engine systems described above, and in others fullyconsistent with this disclosure, pressurizing air or an air/exhaustmixture in a boost tank may cause water vapor to condense inside theboost tank. As shown in FIG. 5, therefore, drain valve 90 is coupled toboost tank 54. The drain valve may be opened as needed to draincondensate from the boost tank. The condensate may be drained onto theroad surface below the vehicle in liquid form, or it may be directed tothe exhaust system of the vehicle, evaporated, and discharged as avapor. In one embodiment, the drain valve may be an electronicallycontrolled, normally closed valve configured to open at the command ofelectronic control system 38. As further described hereinafter, theelectronic control system may be configured to command the draining ofthe condensate at a time when the condensate is unfrozen, and whendraining event is unlikely to be noticed by the occupants of thevehicle. More particularly, the electronic control system may beconfigured to delay opening the drain valve until a noise or speed levelin the vehicle is above a threshold and the condensate is (or ispredicted to be) unfrozen. As such, it is possible that a significantquantity of condensate may accumulate in the boost tank before asuitable opportunity for draining arises.

Accordingly, FIG. 5 shows baffles 92 disposed within boost tank 54. Thebaffles are configured to limit the motion of the condensate in theboost tank—i.e., to limit the degree to which condensate may slosharound inside the boost tank. In this manner, the baffles may reduce thelevel of unwelcome noise associated with the boost tank.

FIG. 5 also shows resistive heating elements 94 arranged near boost tank54 and drain valve 90. In one embodiment, the resistive heating elementsmay be arranged in thermal communication with one or more of the drainvalve, the boost tank, and the condensate within the boost tank. Theresistive heating elements are operatively coupled to voltage source 96,which may be any suitable voltage source of the vehicle in which theboost tank is installed. In one embodiment, the voltage source may be apower grid to which the vehicle is connected when parked, refueled, orserviced. In another embodiment, the voltage source may be a switchablevoltage source linked to a battery of the vehicle. Further, the voltagesource may be switched on or off at the command of electronic controlsystem 38, as necessary to ensure that the drain valve and thecondensate are unfrozen at the intended time of draining.

The embodiments shown in FIGS. 6 and 7 differ from that of FIG. 5 in themanner in which heat is supplied to the condensate and the drain valve.In the embodiment shown in FIG. 6, coolant conduit 97 is arranged nearboost tank 54 and drain valve 90. The coolant conduit is configured tocirculate engine coolant through heat exchanger 98, which may be aradiator of the engine system in which the boost tank is included. Inthe embodiment shown in FIG. 7, exhaust conduit 46 is arranged near theboost tank and the drain valve. The conduits shown in these embodimentsmay be arranged in thermal communication with one or more of the drainvalve, the boost tank, and the condensate within the boost tank.Continuous or controlled, intermittent heating from these conduits mayensure that the drain valve and the condensate in the boost tank areunfrozen at the intended time of draining.

The configurations described above enable various methods for providingair to a combustion chamber of an engine. Accordingly, some such methodsare now described, by way of example, with continued reference to aboveconfigurations. It will be understood, however, that the methods heredescribed, and others fully within the scope of this disclosure, may beenabled via other configurations as well. The methods presented hereininclude various measuring and/or sensing events enacted via one or moresensors disposed in the engine system. The methods also include variouscomputation, comparison, and decision-making events, which may beenacted in an electronic control system operatively coupled to thesensors. The methods further include various hardware-actuating events,which the electronic control system may command selectively, in responseto the decision-making events.

In the configurations described above, compressed air for filling aboost tank may be supplied via a turbocharger compressor under someconditions and via one or more air pumps under other conditions. Suchconfigurations enable control of the relative amount of exhaust gasstored along with the compressed air, inasmuch as air from thecompressor may be diluted with EGR, while air from the air pumps willtypically not be diluted. Further, the air stored in the boost tank maybe discharged in response to at least two conditions: a tip-incondition, where the throttle valve opens suddenly and the compressor isspinning too slowly to provide the desired MAP; and a tip-out condition,where the throttle valve closes suddenly and the available air chargehas more exhaust gas than can be tolerated. In some embodiments, thedesired relative amount of exhaust gas in the air supplied from theboost tank may differ under these two conditions. In particular, it maybe desirable that the air supplied in response to tip-in contain moreexhaust gas than the air supplied in response to tip-out. One way toaddress this issue is to preemptively fill the boost tank with air orwith an air-exhaust mixture in a manner that anticipates operatingconditions when the stored air is later discharged.

Accordingly, FIG. 8 illustrates an example method 100 for selecting asource for filling a boost tank based on engine operating conditions.The method rests on the premise that tip-in is likely to occur when theengine speed is initially high, and tip-out is likely to occur when theengine speed is initially low. Therefore, the method fills the boosttank with EGR-diluted air from the compressor at relatively low enginespeeds and with fresh air from an air pump at relatively high enginespeeds. In this manner, the boost tank is pressurized with air having afirst relative amount of engine exhaust before increasing a throttlevalve opening; such air may be discharged from the boost tank when thethrottle valve opening is increased. Further, the boost tank may bepressurized with air having a second, lower, relative amount of engineexhaust before decreasing the throttle valve opening; such air may bedischarged from the boost tank when the throttle valve opening isdecreased. Accordingly, the relative amount of engine exhaust in the airpressurized in the boost tank may be varied based on engine operatingconditions.

Method 100 begins at 102, where the engine speed is sensed. The enginespeed may be sensed by interrogating any engine-system sensor responsiveto engine speed or a surrogate thereof. Such sensors may include anengine-rotation sensor, a mass air flow sensor, etc. The method thenadvances to 104, where it is determined whether the engine is in alower-speed region. The lower-speed region may correspond to any regionwhere the engine speed is above idle but below a first threshold value.As such, an EGR valve in the engine system may be at least partly open,and the compressor may be inducting at least some exhaust gas in thelower-speed region. When the engine speed is below the first thresholdvalue, it is unlikely that stored boost will be needed to displaceexhaust-diluted air from the intake, but more likely that stored boostwill be needed to avert turbo lag. If it is determined that the engineis in the lower-speed region, then the method advances to 106, whereeffluent of the turbocharger compressor is admitted to and stored in theboost tank. During or prior to admission of such air, an EGR valve thatregulates a flow of engine exhaust to the turbocharger compressor may beadjusted.

In one embodiment, such effluent may be admitted during throttle valveclosure. And in embodiments where an appropriate flow restrictor (flowrestrictor 28, for example) is included in the compressor by-pass,storing may continue even when the by-pass valve of the compressor isopen and when air is flowing through the by-pass. In one embodiment, afill valve may be opened (actively or passively) to admit air to theboost tank.

However, if it is determined at 104 that the engine is not in thelower-speed region, then the method advances to 108, where it isdetermined whether the engine is in a higher-speed region. Thehigher-speed region may correspond to any region where the engine speedis above a second threshold value higher than the first threshold value.When the engine speed is above the second threshold value, it isunlikely that stored boost will be needed to avert turbo lag, but morelikely that stored boost will be needed to displace exhaust-diluted airfrom the intake. Therefore, if it is determined that the engine is inthe higher-speed region, then the method advances to 110, where air froman air pump distinct from the turbocharger compressor is admitted to theboost tank. Following 106, 108, or 110, method 100 returns.

In other embodiments, relative amounts of air admitted from thecompressor and the air pump, as well as the opening amount of the EGRvalve may be varied in any manner whatsoever so that air having agreater relative amount of engine exhaust is stored during lowerengine-speed conditions, and air having a lower relative amount ofengine exhaust is stored during higher engine speed conditions. In oneembodiment, for example, effluent of the turbocharger compressor may bepressurizing and stored in the boost tank when a relative amount ofengine exhaust in an effluent of the compressor is below a threshold.Such air may be stored in the boost tank and discharged duringthrottle-valve closure.

The above method illustrates only one of many contemplated embodimentswhere a decision to admit air from a turbocharger compressor or from anair pump is made based on engine operating conditions. Other methods areenvisaged for determining whether or not, how, and at what level tooperate the air pump and thereby make compressed air available foradmittance to the boost tank. One such method is illustrated in FIG. 9.

FIG. 9 illustrates an example method 112 for operating an air pump basedon such factors as operator engine torque demand, CSER, DFSO, andwhether or not the vehicle is coupled to an external electrical grid.The method begins at 114, where it is determined whether the vehicle isplugged into an external electrical grid. If the vehicle is plugged intothe external electrical grid, then the method advances to 116, where theair pump is operated. In embodiments where the operational level of thepump may be selected from among a plurality of operational levels, thepump may be run at maximum level when the vehicle is plugged into theexternal electrical grid. (Depending on the particular embodiment beingconsidered, the operational level of the air pump may correspond topumping rate, pressurization, air flow, mechanical or electrical loadpresented by the air pump, number of unfueled combustion chambers duringDFSO, etc.) Accordingly, the effluent of the air pump may be admitted tothe boost tank selectively, based on an availability of stationaryelectrical grid power. In another embodiment, the pump may be operatedat this level also when regenerative braking generates electric power inexcess of what the vehicle battery can accept.

However, if it is determined that the vehicle is not plugged into anexternal electrical grid, then method 112 advances to 118, where anoperator engine torque demand in the vehicle is sensed. The operatorengine torque demand may be sensed by interrogating any suitable sensorresponsive to operator engine torque demand, such as a pedal positionsensor. The method then advances to 120, where the operational level ofthe pump is varied in response to the operator engine torquedemand—e.g., increased as operator engine torque demand decreases anddecreased as operator engine torque demand increases. In one embodiment,the operational level may fall to zero when the operator engine torquedemand is at a maximum. In one embodiment, the operational level may beset to provide maximum pressurization when the operator's foot is offthe pedal and the vehicle is moving.

Method 112 then advances to 122, where a CSER condition of the enginesystem is assessed. In one embodiment, assessing the CSER condition maycomprise measuring one or more temperatures—ambient temperature,exhaust-system temperature, catalyst temperature, etc. The method thenadvances to 124, where the operational level of the air pump is variedin response to the CSER condition. This action reflects the fact thatincreased engine load—including increased shaft work—is relativelyinexpensive when the CSER demand is high. In one embodiment, theoperational level of the air pump may be increased at low catalysttemperatures and decreased at high catalyst temperatures. The methodthen advances to 126, where the DFSO mode of the engine system—e.g., thenumber of unfueled combustion chambers—is sensed. The method thenadvances to 128, where the operational level of the pump is varied inresponse to the DFSO mode. For example, the operational level of thepump may be increased when a greater number of combustion chambers ofthe engine are unfueled and decreased when a lesser number of combustionchambers of the engine are unfueled. This action reflects the fact thatextra mechanical energy for operating the air pump is generallyavailable during DFSO conditions, and in engine systems configured tooperate at least one unfueled combustion chamber as an air pump, thoseone or more cylinders will be available for pumping air during DFSOconditions. Following 116 or 128, method 112 returns. It will be notedthat this method effectively provides a pneumatic brake regenerationmechanism, which uses braking energy to compress intake air.

As the method above illustrates, various factors may influence theoperational level of an air pump used to provide air to a boost tank.Other methods address more particular considerations that arise when theair pump is a configurable vacuum/pressure pump. FIG. 10 illustrates onesuch method by way of example.

Method 130 begins at 132, where a vacuum level in a vacuum manifold ofan engine system is sensed or inferred. The method then advances to 134,where it is determined whether the vacuum level sensed or inferred issufficient for vehicle braking. If the vacuum level is not sufficient,then the method advances to 136, where the air pump is configured toevacuate the vacuum reservoir, and to 138, where the air pump isoperated as a vacuum pump. The air pump may be configured to evacuatethe vacuum reservoir by actuating one or more electronically controlledvalves coupled to the air pump, as noted hereinabove in the detaileddescription of FIG. 1.

If the vacuum level is sufficient for vehicle braking, then method 130advances to 140, where the air pump is configured to pressurize theboost tank. In this manner, air from the air pump may be admittedselectively to the boost tank, based on the pressure in the vacuumreservoir. The air pump may be configured to pressurize the boost tankby actuating one or more electronically controlled valves coupled to thepump, as noted hereinabove. The method then advances to 142, where it isdetermined whether the boost tank is filled to a desired pressure. Ifthe boost tank is filled to the desired pressure, then the methodreturns to 136. In this state, the air pump is configured to maintainthe vacuum of the vacuum manifold and presents minimal mechanical orelectrical load on the engine system. However, if the boost tank is notfilled to the desired level, the method advances to 144, where the airpump is operated as a pressure pump. Following 138 or 144, the methodreturns.

The methods described above relate to filling of the boost tank undervarious conditions. Other methods relate to controlling the manner inwhich compressed air is discharged from the boost tank to improve engineoperation.

FIG. 11 illustrates an example method 146 for discharging compressed airfrom a boost tank in one embodiment. The method may be enacted inresponse to full or partial throttle valve opening, or under otherconditions. Method 146 begins at 148, where the engine system TIP issensed. The TIP may be sensed via a suitable pressure sensor coupledupstream of the throttle valve, or it may be inferred based on otherfactors. The method then advances to 150, where it is determined whetherthe sensed TIP is less than the desired MAP for current operatingconditions. If TIP is less than the desired MAP, then the methodadvances to 152, where the pressure in the boost tank is sensed;otherwise, the method returns. The pressure in the boost tank may besensed by interrogation of a pressure sensor coupled to the boost tank,or in any other suitable manner. In one embodiment, the air in the boosttank may be discharged once the throttle is fully open, and when thetank pressure is above TIP. In this case, a fully open throttle is takento mean MAP<MAP_desired.

Continuing in FIG. 11, method 146 then advances to 154, where it isdetermined whether the boost tank pressure is greater than the desiredMAP. If the boost tank pressure is greater than the desired map, thenthe method advances to 156, where the relative EGR amount in the boosttank is sensed; otherwise the method returns. In one embodiment, therelative EGR amount may be sensed via a sensor (e.g., a humidity sensoror carbon dioxide sensor). In another embodiment, the relative EGRamount may be inferred or recalled from a memory of an electroniccontrol system rather than sensed per se. The method then advances to158, where it is determined whether the sensed EGR amount is below adesired range. If the sensed relative EGR amount is below the desiredrange, then the method advances to 160, where a level of spark retardapplied in the combustion chambers is increased; otherwise the methodadvances to 164, where the boost tank is finally open to the intakemanifold. This action causes the air pressurized in the boost tank to bedischarged to the intake manifolds. The method then advances to 162,where the throttle valve opening amount is reduced to compensate for theincreased oxygen content in the air charge being supplied to the intakevia the boost tank, and thento 164. Note that the air charge supplied tothe combustion chambers under these conditions may include less than theexpected amount of EGR. Following 164, method 146 returns.

The foregoing methods illustrate example scenarios for filling a boosttank and for discharging air from the boost tank depending on operatingconditions of the engine system. The following method demonstrates thatsome conditions are envisaged where throttle valve closure triggers afilling of the boost tank, and other conditions are envisaged wherethrottle valve closure triggers a discharge of air from the boost tank.

FIG. 12 illustrates an example method 166 for responding to throttlevalve closure in one embodiment. The method begins at 168, where athrottle valve opening amount is sensed. The method then advances to170, where it is determined whether the throttle valve opening amount isdecreasing faster than a threshold rate. If the throttle valve openingamount is decreasing faster than the threshold rate, then the methodadvances to 172, where the pressure in the boost tank is sensed;otherwise, the method returns. The method then advances to 174, wherethe boost tank is opened to the intake manifold. If the pressure of theair in the boost tank was greater than the MAP at the time of opening,then this action causes the air pressurized in the boost tank to bedischarged to the intake manifold. Accordingly, at least some of the airstored in the boost tank may be discharged to the intake manifoldconcurrent with throttle closure. The boost tank may be opened to theintake manifold actively (e.g., by actuating an electronicallycontrolled valve) or passively (e.g., by pressure-induced cracking of acheck valve). The method then advances to 176, where it is determinedwhether the boost tank pressure was above a threshold value at the timeof sensing. If the boost tank pressure was above the threshold value atthe time of sensing, then the method advances to 178, where the wastegate of the turbine is opened, and to 180, where by-pass valve of thecompressor is opened. These actions allow EGR-diluted air from theintake to be displaced and flushed out by air from the boost tank.

However, if it is determined at 176 that the boost tank pressure was notabove the threshold value at the time of sensing, then the methodadvances to 182, where a delay is executed. The delay may providesufficient time for compressed air from the turbocharger compressor tobe admitted to the boost tank. The delay may be a fixed or variabledelay. In one embodiment, a variable delay may be applied, and mayextend until the pressure in the boost tank has risen to a desiredvalue, or, has approached a recommended pressure rating of the boosttank. In another embodiment, the delay may be extended until therelative amount of exhaust gas in the air pressurized in the boost tankapproaches a desired value. In yet another embodiment, during or inadvance of the delay, an EGR valve that regulates a flow of engineexhaust to the turbocharger compressor may be adjusted. In this manner,the relative amount of engine exhaust in the air pressurized in theboost tank may be varied based on engine operating conditions. As aresult of the delay, the boost tank may be open to the compressor outletduring full throttle valve closure. Thus, the boost tank may absorb thehigh pressure spike that occurs on full throttle valve closure, therebymaximizing air pressure in the boost tank. Further, effluent of thecompressor may be admitted to the boost tank when a waste gate of theturbine is closed, so that the boost tank is filled from a TIP sourcegreater than MAP. Following 182, the method returns to 178 and to 180,where the waste gate and compressor by-pass valves are finally opened.

Configurations that include an auxiliary throttle valve in addition to amain throttle valve enable all of the control functions describedhereinabove, and others as well. For example, air from a compressorand/or an air pump may be stored in a boost tank and subsequentlydischarged to an intake manifold under various operating conditions.These include conditions of increased opening of the main throttlevalve, tip-in, conditions of decreased opening of the main throttlevalve, and tip-out. In one embodiment, air having a first relativeamount of exhaust gas may be stored during a first operating conditionand discharged in response to increased opening of the main throttlevalve. Further, air having a second, lower, relative amount of exhaustgas may be stored during a second operating condition and discharged inresponse to closure of the main throttle valve. In this example, thesecond operating condition may be characterized by a greater enginespeed than the first operating condition.

Other embodiments are contemplated as well. For example, instead of themethod shown in FIG. 12, the boost tank can simply be filled when excessboost is available, and when the pressure inside the boost tank is belowa threshold. In this case, excess boost may be available whenever thecompressor bypass valve would otherwise be commanded open, whenever thewastegate would otherwise be commanded open, or whenever TIP>MAP+a smallpressure margin. Such conditions could determine inter alia when to tryuse compressor boost to fill the boost tank. Further control finessecould be applied to affect conditions to purposely and opportunisticallyprovide the excess boost conditions.

FIG. 13 illustrates an example method 184 for regulating an air supplyto an intake manifold via main and auxiliary throttle valves in oneembodiment. The method may be enabled by an engine system having mainand auxiliary throttle valves, such as engine system 80 of FIG. 4. Themethod may be enacted whenever an electronic control system of thevehicle requires a change in manifold air flow. In one embodiment, themethod is enacted when the main throttle valve is opened, so that airfrom the compressor is admitted to the intake manifold.

Method 184 begins at 186, where an air flow rate through a main throttlevalve is sensed. The manifold air flow may be sensed by interrogation ofa manifold air flow (MAF) sensor or in any other suitable manner. Themethod then advances to 188, where it is determined whether the sensedair flow rate is less than a target value of the MAF (based on operatorengine torque demand, engine load, etc.). If the sensed air flow rate isnot less than the target value, then the method returns. Otherwise, themethod advances to 190, where a pressure differential between acompressor outlet and a secondary inlet of an air ejector coupledthereto is sensed. The method advances to 192, where an auxiliarythrottle valve opening amount is adjusted based on the sensed pressuredifferential. In this manner, some of the air stored in the boost tankmay be discharged to the intake manifold via the auxiliary throttlevalve. In one scenario, this action may displace EGR-diluted air in theintake manifold in response to tip-out and/or full or partial throttleclosure, for example. Such displacement is further enabled inembodiments where the main throttle valve is meanwhile held open, sothat the EGR-diluted air may be released from the intake manifold. Inone embodiment, the auxiliary throttle valve opening may be increasedwhen the air ejector secondary inlet pressure is lower than thecompressor outlet pressure. In other scenarios, where displacement ofEGR-diluted air from the intake manifold is not desired, the mainthrottle valve may be closed during or before discharging the stored airvia the auxiliary throttle valve to prevent reverse air flow through themain throttle valve. Accordingly, stored air may be discharged via theauxiliary throttle valve when the TIP is below a threshold, at tip-in,and/or in response to full or partial main throttle valve opening. Inthese and other embodiments, the stored air may be discharged via theauxiliary throttle valve while additional air is drawn into the intakemanifold via the air ejector coupled between the boost tank and theintake manifold. Further, the auxiliary throttle valve opening amountmay be decreased when the air ejector secondary inlet pressure isgreater than the compressor outlet pressure. In one embodiment, thestored air may be discharged through a pressure recovery cone coupledbetween the air ejector and the intake manifold. In another embodiment,where a check valve is coupled upstream of the auxiliary throttle valve,some or all of the control elements of this method may be enactedpassively. Following 192, the method returns.

The foregoing method shows inter alia how the opening amount of anauxiliary throttle valve in a suitably configured engine system may becontrolled to emulate a passive check valve. Other methods are envisagedin which a main throttle valve in a suitably configured engine systemmay be actively controlled to emulate a passive check valve.

FIG. 14 illustrates an example method 194 for ensuring unidirectionalair flow through a main throttle valve in a suitably configured enginesystem. In such a system, the main throttle valve may have no checkvalve coupled in series with it, and may thereby be configured to admitair flow in a forward direction and in a reverse direction. The methodbegins at 196, where a corrected flow potential across the main throttlevalve is sensed. The corrected flow potential may be sensed byinterrogating one or more sensors coupled in the engine system. In oneembodiment, the corrected flow potential may correspond toTIP-MAP-DELTA, where TIP is the throttle inlet pressure upstream of themain throttle valve, MAP is the manifold air pressure, and DELTAcorresponds to a relatively small ‘cracking’ pressure—e.g., 1 pound persquare inch. The method then advances to 198, where it is determinedwhether the corrected flow potential is positive—viz., whether it isoriented to cause air to flow through the throttle valve toward theintake manifold. If it is determined that the corrected flow potentialis positive, then the method advances to 200, where the opening amountof the throttle valve is adjusted (increased or decreased) in order toregulate the MAF to the desired level. However, if the corrected flowpotential is not positive—viz., if it is oriented to cause air to flowthrough the throttle valve from the intake manifold—then the methodadvances to 202, where the throttle valve opening amount is decreased toregulate or shut off air flow through the main throttle valve. Following200 or 202, the method returns.

FIG. 15 illustrates an example method 204 for releasing condensate froma boost tank in a vehicle when the occupants of the vehicle are unlikelyto notice. The method begins at 206, where it is determined whether thepredicted amount of condensate accumulation in the boost tank is above athreshold. If the predicted amount of condensate accumulation is notabove the threshold, then the method returns. Otherwise, the methodadvances to 208, where it is determined whether the vehicle speed isabove a threshold. If the vehicle speed is not above the threshold, thenthe method returns. Otherwise, the method advances to 210, where it isdetermined whether condensate freezing conditions are predicted. Ifcondensate freezing conditions are predicted, then the method advancesto 212, where the boost tank and a drain valve or other appliance fordraining the boost tank are actively heated. The condensate and drainvalve may be heated by energizing an electrical heating element inthermal communication with the condensate or by flowing a heated fluidsuch as engine exhaust or engine coolant through a conduit in thermalcommunication with the condensate and drain valve, for example. Afterthis action, the method then returns to 206. In this manner, the releaseof the condensate from the boost tank may be delayed until thecondensate is unfrozen. However, if it is determined that condensatefreezing conditions are not predicted, then the method advances to 214,where such heating is turned off. From 214, the method advances to 216,where a normally closed, electronically controlled valve configured todrain condensate from the boost tank is opened. Following this action,the method returns. Thus, the condensate may be released from the boosttank when the condensate is unfrozen and when a noise level of thevehicle is above a noise threshold. In this manner, the condensate maybe released easily and without drawing the attention of the occupants ofthe motor vehicle. In the illustrated embodiment, vehicle speed is usedas a surrogate or predictor of noise level. It will be appreciated,however, that other surrogates and predictors of noise level are equallyapplicable.

In another embodiment, the condensate may be actively pumped out of theboost tank, not merely drained from the boost tank under the force ofgravity. In yet another embodiment, the condensate may be forced out ofthe boost tank due to the pressure of the compressed air stored therein.

It will be understood that the example control and estimation routinesdisclosed herein may be used with various system configurations. Theseroutines may represent one or more different processing strategies suchas event-driven, interrupt-driven, multi-tasking, multi-threading, andthe like. As such, the disclosed process steps (operations, functions,and/or acts) may represent code to be programmed into computer readablestorage medium in an electronic control system.

It will be understood that some of the process steps described and/orillustrated herein may in some embodiments be omitted without departingfrom the scope of this disclosure. Likewise, the indicated sequence ofthe process steps may not always be required to achieve the intendedresults, but is provided for ease of illustration and description. Oneor more of the illustrated actions, functions, or operations may beperformed repeatedly, depending on the particular strategy being used.

Finally, it will be understood that the articles, systems and methodsdescribed herein are exemplary in nature, and that these specificembodiments or examples are not to be considered in a limiting sense,because numerous variations are contemplated. Accordingly, thisdisclosure includes all novel and non-obvious combinations andsub-combinations of the various systems and methods disclosed herein, aswell as any and all equivalents thereof.

The invention claimed is:
 1. A method for providing air to an engine including a compressor and a boost tank selectably coupled to an intake manifold, comprising: pressurizing and storing air in the boost tank during excess boost conditions including pressurizing and storing gas when a relative amount of engine exhaust in an effluent of the compressor is below a threshold; and discharging air stored in the boost tank to the intake manifold concurrent with throttle closure of an intake-air throttle coupled between the intake manifold and the compressor.
 2. The method of claim 1, further comprising discharging some of the air stored in the boost tank to the intake manifold in response to throttle opening.
 3. The method of claim 2, wherein pressurizing and storing air in the boost tank comprises varying a relative amount of engine exhaust in the air pressurized in the boost tank based on engine operating conditions, and wherein varying the relative amount of engine exhaust comprises storing air having a greater relative amount of engine exhaust during lower engine-speed conditions and storing air having a lower relative amount of engine exhaust during higher engine-speed conditions.
 4. The method of claim 1, further comprising storing combustion exhaust gas from fueled cylinders in the boost tank during selected conditions.
 5. The method of claim 1, further comprising varying a relative amount of engine exhaust from fueled cylinders in air pressurized in the boost tank based on engine operating conditions.
 6. A method for an engine, comprising: pressurizing air via a turbocharger compressor; storing the pressurized air in a boost tank coupled to an intake manifold during excess boost conditions; discharging the stored air to the intake manifold concurrent with closure of an intake-air throttle coupled between the intake manifold and the turbocharger compressor; and varying a relative amount of engine exhaust from fueled cylinders in the stored air based on engine operating conditions.
 7. The method of claim 6, further comprising discharging some of the stored air stored to the intake manifold in response to opening of the intake-air throttle.
 8. The method of claim 7, wherein varying the relative amount of engine exhaust comprises storing air having a greater relative amount of engine exhaust during lower engine-speed conditions and storing air having a lower relative amount of engine exhaust during higher engine-speed conditions.
 9. The method of claim 6, wherein pressurizing and storing the air in the boost tank comprises pressurizing and storing gas when a relative amount of engine exhaust in an effluent of the compressor is below a threshold.
 10. The method of claim 6, further comprising storing combustion exhaust gas from fueled cylinders in the boost tank during selected conditions.
 11. A method for an engine including a compressor and a boost tank selectably coupled to an intake manifold, comprising: pressurizing and storing air in the boost tank during excess boost conditions; discharging air stored in the boost tank to the intake manifold concurrent with throttle closure of an intake-air throttle coupled between the intake manifold and the compressor; and storing combustion exhaust gas from fueled cylinders in the boost tank during the excess boost conditions, wherein pressurizing and storing the air in the boost tank comprises pressurizing and storing gas when a relative amount of engine exhaust in an effluent of the compressor is below a threshold.
 12. The method of claim 11, further comprising discharging some of the air stored in the boost tank to the intake manifold in response to throttle opening.
 13. The method of claim 12, wherein pressurizing and storing air in the boost tank comprises varying a relative amount of engine exhaust in the air pressurized in the boost tank based on engine operating conditions, and wherein varying the relative amount of engine exhaust comprises storing air having a greater relative amount of engine exhaust during lower engine-speed conditions and storing air having a lower relative amount of engine exhaust during higher engine-speed conditions.
 14. The method of claim 11, further comprising varying a relative amount of engine exhaust from fueled cylinders in air pressurized in the boost tank based on engine operating conditions. 